[libc++] Ensure that we restore invariants in basic_filebuf::overflow

libcxx/include/fstream

@@ -821,6 +821,14 @@ typename basic_filebuf<_CharT, _Traits>::int_type basic_filebuf<_CharT, _Traits>
 
 template <class _CharT, class _Traits>
 typename basic_filebuf<_CharT, _Traits>::int_type basic_filebuf<_CharT, _Traits>::overflow(int_type __c) {
+  auto __failed = [this]() {
+    if (this->pptr() == this->epptr() + 1) {
+      this->pbump(-1); // lose the character we overflowed above -- we don't really have a
+                       // choice since we couldn't commit the contents of the put area
+    }
+    return traits_type::eof();
+  };
+
   if (__file_ == nullptr)
     return traits_type::eof();
   __write_mode();
@@ -841,8 +849,9 @@ typename basic_filebuf<_CharT, _Traits>::int_type basic_filebuf<_CharT, _Traits>
 
   if (__always_noconv_) {
     size_t __n = static_cast<size_t>(this->pptr() - this->pbase());
-    if (std::fwrite(this->pbase(), sizeof(char_type), __n, __file_) != __n)
-      return traits_type::eof();
+    if (std::fwrite(this->pbase(), sizeof(char_type), __n, __file_) != __n) {
+      return __failed();
+    }
   } else {
     if (!__cv_)
       std::__throw_bad_cast();
@@ -854,34 +863,38 @@ typename basic_filebuf<_CharT, _Traits>::int_type basic_filebuf<_CharT, _Traits>
     char* __extbuf_end = __extbuf_;
     do {
       codecvt_base::result __r = __cv_->out(__st_, __b, __p, __end, __extbuf_, __extbuf_ + __ebs_, __extbuf_end);
-      if (__end == __b)
-        return traits_type::eof();
+      if (__end == __b) {
+        return __failed();
+      }
 
       // No conversion needed: output characters directly to the file, done.
       if (__r == codecvt_base::noconv) {
         size_t __n = static_cast<size_t>(__p - __b);
-        if (std::fwrite(__b, 1, __n, __file_) != __n)
-          return traits_type::eof();
+        if (std::fwrite(__b, 1, __n, __file_) != __n) {
+          return __failed();
+        }
         break;
 
         // Conversion successful: output the converted characters to the file, done.
       } else if (__r == codecvt_base::ok) {
         size_t __n = static_cast<size_t>(__extbuf_end - __extbuf_);
-        if (std::fwrite(__extbuf_, 1, __n, __file_) != __n)
-          return traits_type::eof();
+        if (std::fwrite(__extbuf_, 1, __n, __file_) != __n) {
+          return __failed();
+        }
         break;
 
         // Conversion partially successful: output converted characters to the file and repeat with the
         // remaining characters.
       } else if (__r == codecvt_base::partial) {
         size_t __n = static_cast<size_t>(__extbuf_end - __extbuf_);
-        if (std::fwrite(__extbuf_, 1, __n, __file_) != __n)
-          return traits_type::eof();
+        if (std::fwrite(__extbuf_, 1, __n, __file_) != __n) {
+          return __failed();
+        }
         __b = const_cast<char_type*>(__end);
         continue;
 
       } else {
-        return traits_type::eof();
+        return __failed();
       }
     } while (true);
   }

libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/overflow.writefail.pass.cpp

@@ -0,0 +1,72 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: no-filesystem
+
+// setrlimit(RLIMIT_FSIZE) seems to only work as intended on Apple platforms
+// REQUIRES: target={{.+}}-apple-{{.+}}
+
+// <fstream>
+
+// Make sure that we properly handle the case where we try to write content to a file
+// but we fail to do so because std::fwrite fails.
+
+#include <cassert>
+#include <csignal>
+#include <cstddef>
+#include <fstream>
+#include <string>
+
+#include "platform_support.h"
+#include "test_macros.h"
+
+#if __has_include(<sys/resource.h>)
+#  include <sys/resource.h>
+void limit_file_size_to(std::size_t bytes) {
+  rlimit lim = {bytes, bytes};
+  assert(setrlimit(RLIMIT_FSIZE, &lim) == 0);
+
+  std::signal(SIGXFSZ, [](int) {}); // ignore SIGXFSZ to ensure std::fwrite fails
+}
+#else
+#  error No known way to limit the amount of filesystem space available
+#endif
+
+template <class CharT>
+void test() {
+  std::string temp = get_temp_file_name();
+  std::basic_filebuf<CharT> fbuf;
+  assert(fbuf.open(temp, std::ios::out | std::ios::trunc));
+
+  std::size_t const limit = 100000;
+  limit_file_size_to(limit);
+
+  std::basic_string<CharT> large_block(limit / 10, CharT(42));
+
+  std::streamsize ret;
+  std::size_t bytes_written = 0;
+  while ((ret = fbuf.sputn(large_block.data(), large_block.size())) != 0) {
+    bytes_written += ret;
+
+    // In theory, it's possible for an implementation to allow writing arbitrarily more bytes than
+    // set by setrlimit, but in practice if we bust 100x our limit, something else is wrong with the
+    // test and we'd end up looping forever.
+    assert(bytes_written < 100 * limit);
+  }
+
+  fbuf.close();
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}